Test out superpixel-based manual correction technique

[joshuacwnewton]

NB: This issue has been migrated from the spinalcordtoolbox repository: https://github.com/spinalcordtoolbox/spinalcordtoolbox/issues/3129

Link: https://eoss-image-processing.github.io/2020/12/21/3-dimensional-segmentation.html

This effort would be closely tied to the existing manual correction efforts found in:

• https://github.com/spinalcordtoolbox/spinalcordtoolbox/issues/3353
• https://github.com/spinalcordtoolbox/manual-correction